Although they might deny it, many of the politicians are asking people to vote tactically, sorry I think they may have called it intelligently, to ensure that whoever they are gains the most seats. Sometimes, they say, you need to vote to keep people out.
I think that is a load of absolute bollocks. As far as I see it your vote represents what you believe in. Who you choose to vote for demonstrates your broad agreement with their policies (or most of their policies, no one party is perfect after all). If your vote is used to 'Get Labour Out' or to 'Stop the Tories Getting In' all you are doing is playing into the politicians hands and wasting the opportunity to say 'I agree with this'.
Take voting Green for example. They have no chance of winning the election. But should you choose to vote for the Green Party then you are saying that the Environment is an incredibly important issue to you. If everyone who believes that votes for the Green Party, they still won't win, but they will get enough support that will make the other parties think 'huh, this environment issue thing is obviously important, how can we attract those voters by including it into our own policies'. But if no one voted for the Green Party because they didn't have a chance of winning how would the politicians ever know that environmental issues are something that matters to us?
Likewise with the big three parties. Whoever you vote for you are making a statement. If you vote for a party you don't agree with what statement are you making? That you agree with their policies. Whichever way you look at it this election is going to be close. The politicians are going to have to look at each others policies and work out a way to represent us all. You should make your vote count by making a positive statement and voting for what you believe.